Process and device for dewatering a fiber web

ABSTRACT

A process and a device for dewatering a fiber web  3,  especially a paper web, using vacuum. The vacuum seen by a given unit are of the web is pulsed between a maximum value V and a minimum value. At least one suction box  7  is provided with perforations beneath the running web where the vacuum is applied with a pulsating effect on the web.

BACKGROUND

The invention concerns a process and a device for dewatering a fiber web, especially paper web, using vacuum.

In known processes, dewatering of, e.g., a paper web on an air-permeable wire or felt takes place after the forming zone with vacuum that extracts water from the web and the wire or felt through boxes located under the web and the wire. Vacuum dewatering is used particularly in tissue machines working according to the through drying principle. The water is normally extracted through slots in a plate over which the paper web passes. The goal of the dewatering is to achieve as high a dryness of the paper web as possible after application of the vacuum. However, the achievable dryness is limited by the intensity of the vacuum and the slot width.

SUMMARY

The goal of the invention is, therefore, to increase the dryness after vacuum dewatering by a few percentage points in comparison with conventional dewatering or to maintain the dryness and achieve energy savings at the vacuum pumps.

The disclosure is characterized by the vacuum applied to each unit of the web, pulsating between a maximum and a minimum value, preferably with the maximum value of the vacuum being between 150 and 900 mbar, most preferably between 300 and 700 mbar. With this pulsating application of the vacuum, dewatering can be greatly improved in comparison with conventional constant application.

An advantageous embodiment is characterized by 3 to 100 pulses, preferably between 25 and 100 pulses, being applied to a particle of the moving paper web. Improved dewatering performance is obtained by a higher number of pulses.

An advantageous further embodiment is characterized by the minimum value of the vacuum being 0 mbar.

To push the dewatering performance further, it is advantageous to use two or more vacuum sources successively for dewatering the fiber web, especially paper web.

A pulsation frequency of between 1,500 and 10,000 Hertz has proved particularly advantageous for the dewatering performance.

The disclosure is also directed to a device for dewatering a fiber web, especially a paper web, using vacuum. It is characterized by at least one suction box being provided, at which a vacuum is pulsating between a maximum and a minimum value, with the—at least one—suction box having a perforated plate at the side facing the fiber, especially paper, web. With such a suction box connected to a vacuum plant, essentially higher dryness values can be achieved than with conventional plants.

An advantageous embodiment is characterized by the holes of the perforated plate being circular, oblong or elliptic. High free areas can be obtained in this manner.

An advantageous further embodiment is characterized by the holes of the perforated plate having a distance (between centres) of 4 to 25 mm viewed in the direction of the web run, with the holes in successive rows of holes crosswise to the direction of the web run possibly overlapping. With such a pitch, an optimum number of pulses can be realised.

If the—at least one—suction box has a length of 100 to 500 mm, preferably 200 to 400 mm, viewed in the direction of the web run, a sufficient number of pulses and, therefore, high dewatering performance can be realised.

It is especially advantageous to have several, especially two, suction boxes arranged successively at the fiber web, especially paper web, in the direction of the web run.

BRIEF DESCRIPTION OF THE DRAWING

The preferred embodiments of the invention are described below in examples and with reference to the drawing, where

FIG. 1 shows an arrangement on a forming wire of a through drying machine according to an aspect of the invention;

FIG. 2 shows a vacuum box according to an aspect of the invention;

FIG. 3 shows the course of the vacuum for a particle of the paper web; and

FIG. 4 is a top view of a perforated plate according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows the wet section of a paper machine, particularly of a through drying plant, in a tissue machine. This section is designed for sheet formation and comprises a forming wire 1 and an outer wire 2. The pulp suspension is fed in between the forming wire 1 and the outer wire 2 through nozzles by a headbox 5 and led around a forming roll 6, which may contain suction elements, whereby a paper web 3 forms. This paper web 3 is led over at least one, preferably several, vacuum boxes 7, which can also be arranged opposite deflection rolls 8. Subsequently, paper web 3 is taken over by a dryer fabric 4 using a web removal box 9 or a web removal roll. From the dryer fabric 4 the web is fed to the dryer, especially through dryer.

FIG. 2 shows a very small unit area e.g., (particle) 10 of the paper web 3 being moved over a perforated plate 11 together with the forming wire 1. Thus, the vacuum applied at the vacuum box 7 acts on particle 10 pulsatingly. From pitch A [m] of the holes 12 and the machine speed S [m/s], the pulse frequency f [Hz] can be calculated using the following formula: f=S/A. The holes 12 of the perforated plate 11 can be of circular, oblong, or elliptic shape.

An increase of approx. 25% to approx. 28% of the dryness of a web has been achieved with an arrangement according to the invention and with the same open hole area as the conventional perforations of a suction box and identical suction volume.

FIG. 3 shows the course of the vacuum acting on a particle of the paper web as a function of time. The value V may amount to between 150 and 900 mbar especially between 300 and 700 mbar.

FIG. 4 shows a top view of a perforated plate 11 according to the invention, where the holes 12 have pitch A in the direction of the web run (machine direction, MD). It can also be seen that the holes 12 of successive rows in crosswise direction to the web run (machine direction) overlap and thus no strip remains that is not subjected to the vacuum. 

1. In a process for dewatering a fiber web, by applying a vacuum to each unit area of the web, the improvement comprising that the vacuum applied to each unit area of the web is pulsed between a maximum value and a minimum value.
 2. Process according to claim 1, wherein the web moves in a machine direction and between 3 to 100 pulses are applied to each unit area of the moving web.
 3. Process according to claim 1, wherein the maximum value (V) of the vacuum is between 150 and 900 mbar.
 4. Process according to claim 3, wherein the maximum value (V) of the vacuum is between 500 and 700 mbar.
 5. Process according to claim 1, wherein the minimum value of the vacuum is 0 mbar.
 6. Process according to claim 1, wherein two distinct vacuum devices dewater the web successively.
 7. Process according to claim 2, wherein the frequency of the pulsations is between 1,500 and 10,000 Hertz.
 8. Process according to claim 1, wherein the web is a paper web.
 9. Process according to claim 8, wherein the web moves in a machine direction over a vacuum dewatering device and between 25 and 100 pulses are applied to each unit area of the moving web by said device.
 10. Process according to claim 8, wherein the maximum value (V) of the vacuum is between 150 and 900 mbar.
 11. Process according to claim 8, wherein the minimum value of the vacuum is 0 mbar.
 12. Process according to claim 8, wherein two distinct vacuum devices dewater the web successively.
 13. Process according to claim 9, wherein the web moves in a machine direction over a vacuum dewatering device and between 25 and 100 pulses are applied to each unit area of the moving web by said device.
 14. A device for vacuum dewatering a fiber web moving in a machine direction, wherein the improvement comprises at least one suction box that applies a pulsating vacuum between a maximum value (V) and a minimum value to the fiber web.
 15. Device according to claim 14, wherein the at least one suction box has a perforated plate facing the fiber web.
 16. Device according to claim 15, wherein the perforations are circular, oblong, or elliptic holes.
 17. Device according to claim 15, wherein the holes of the perforated plate have a distance between centres of 4 to 25 mm viewed in the machine direction.
 18. Device according to claim 16, wherein the holes in successive rows in crosswise direction to the machine direction, overlap.
 19. Device according to claim 14, wherein the at least one suction box has a length of 100 to 500 mm, viewed in the machine direction.
 20. Device according to claim 14, wherein a plurality of suction boxes are arranged successively at the fiber web in the machine direction.
 21. Device according to claim 14, wherein the device is situated beneath a moving paper web and the suction box has a perforated plate facing the web.
 22. Device according to claim 21, wherein the perforations are circular, oblong, or elliptic holes.
 23. Device according to claim 22, wherein the holes of the perforated plate have a distance between centres of 4 to 25 mm viewed in the machine direction.
 24. Device according to claim 23, wherein the holes in successive rows in crosswise direction to the machine direction, overlap.
 25. Device according to claim 24, wherein the at least one suction box has a length of 100 to 500 mm, viewed in the machine direction. 